This invention relates to the use of polymeric additives providing increased plasticity, density and strength in the formation of ceramic articles. In particular the present invention involves the use of selected polymer additives together with mineral matrix materials to provide ceramic green bodies, such as those used in sanitaryware articles, having improved green strength and reduced loss to waste byproduct. In addition, the selected polymer additives facilitate handling at processing stages prior to green body formation by enabling high solids ceramic particle slurries to be prepared having viscosities controllable within a selected range.
Ceramic materials are typically prepared by mixing powdered ceramic oxides in an aqueous slurry together with processing aids, such as dispersants and binders. The slurry may be dewatered to produce ceramic particles, after which the particles are aggregated (compacted) into a "green body" having a desired shape; alternatively, the slurry may be charged to a mold where combined shape forming and dewatering occurs to form the green body. Green bodies are subjected to heat treatment (sintering) to convert the green body into a "fired ceramic" having satisfactory strength and durability characteristics for use in a variety of products, for example, chromatographic media, grinding aids, abrasives, catalysts, adsorbents, tableware, tiles, electronic components, construction components, machine components, and particularly sinks, spas, bathroom fixtures, sanitaryware and architectural articles.
The properties of the green bodies generally affect the properties of the final ceramic product. If the green density of the green body is too low, the mechanical properties of the article, such as hardness and toughness, will diminish. If the green strength is too low, it becomes difficult or impossible to process the green body. Thus, it is desirable to provide ceramic green bodies with sufficient green densities and green strengths. Green density is determined by how well the ceramic materials are compacted during processing.
Lack of plasticity in ceramic particles results in increased hardness of the green body. Increased hardness reduces compaction of the particles during green body formation, and therefore reduces the density of the green bodies after compaction. Low green density results in a low density after sintering which reduces mechanical strength of the final ceramic product.
Common plasticizers have been used to improve green body properties; and these include water, ethylene glycols, polyethylene glycols, glycerol, dibutyl phthalate and dimethyl phthalate (James S. Reed, Principles of Ceramic Processing, Second Ed., John Wiley and Sons, p 204, New York, 1995). These plasticizers are either water-soluble or water-insoluble. Water-insoluble plasticizers are difficult to incorporate into ceramic slurries that are water based. The water-soluble plasticizers are also less desirable since they are sensitive to changes in humidity, that is, they are hygroscopic. Thus, ceramic green bodies made by processes using these water-soluble plasticizers exhibit variability in compaction, green density, green strength, shrinkage and die sticking as the humidity varies (Whitman et al., "Humidity Sensitivity of Dry Press Binders," Paper No. SXVIIb-92-94, presented at the 96.sup.th Annual Meeting of the American Ceramic Society, Indianapolis, Ind., Apr. 25, 1994). In addition, since the aforementioned plasticizers are either small molecules or very low molecular weight polymers, they provide little or no adhesion for ceramic powders during processing. The ceramic green bodies processed with these types of plasticizers have reduced strength.
One method for increasing compaction, thus increasing green density and green strength of ceramic green bodies, is to use a binder as a processing aid. U.S. Pat. No. 5,487,855 discloses binders for use in ceramic materials based on water-soluble hydrolyzed copolymers made from monomers having ester or amide functional groups. U.S. Pat. No. 5,908,889 discloses the use of water-soluble polyamides made by condensation polymerization as binders for ceramic materials. Because they are water-soluble, these binders have the disadvantage of being sensitive to changes in humidity and the potential for causing variability in properties during processing.
U.S. Pat. No. 4,968,460 discloses several classes of emulsion polymers useful as binders for ceramic materials, including acrylate ester polymers. These binders provide increased green strength and green density to the ceramic green bodies when the green body is subjected to an energy treatment step, such as electron beam irradiation, X-ray irradiation, ultraviolet radiation, heating treating from 50.degree. C. to 200.degree. C. or combinations of heat and pressure; the additional treatments add time and cost during production of ceramic green bodies.
The present invention seeks to overcome problems involving the use of prior art additives in preparing ceramic compositions by using selected polymer additives that provide ceramic particle slurries having controlled viscosity characteristics, thus allowing the desired additives to be formulated into the ceramic composition without detracting from the handling properties of the slurries used to prepare green bodies, especially where processing steps involving slurry storage, slurry transfer, spray drying, slip casting or filter pressing are involved.